Spacers are utilized in nuclear fuel bundles in order to maintain the required spacing between vertically upstanding side-by-side fuel rods within the bundles. A review of the construction of nuclear fuel bundles can clarify the function of the spacers set forth herein.
Simply stated, a nuclear fuel bundle contains many so-called fuel rods supported on a lower tie plate. This lower tie plate is for supporting and spacing in side-by-side relation the rods of the bundle. These supported fuel rods extend upwardly of the fuel bundle to an upper tie plate.
The fuel bundle between the tie plates is surrounded by a fuel channel which channel is usually square in cross section. This channel surrounds the fuel rods and extends from the lower tie plate to and beyond the upper tie plate.
Fluid flow occurs within the fuel bundle through the lower tie plate, between the fuel rods, and out through the upper tie plate. Water coolant enters through the lower tie plate and a mixture of steam and water exits through the upper tie plate.
Fuel rods are typically of the order of 170" long, whereas the fuel rod diameter is typically between 0.35" and 0.50". This being the case, it is necessary to brace the fuel rods in between the tie plates with spacers.
The primary function of a spacer is to support the fuel rods and hold them in their correct side-by-side relation. A secondary function of the spacers is to enhance the water film on the fuel rods and to improve the critical power performance. Simply stated, by maintaining a water film on the fuel rods, the individual rods are maintained in an optimum steam generating condition and have resistance to so-called "non-nucleate boiling" which can cause overheating and damage to the individual fuel rods.
Spacers are commonly constructed with two constituent parts. The first constituent part includes an interior grid. In one type of spacer, this grid consists of an array of ferrules which ferrules surround each of the fuel rods and keep the fuel rods spaced apart one from another. The second constituent part of the spacer is an outer band. This band extends around the grid and locates the grid inside the channel. The band occupies some of the gap between the grid and the channel, and limits the coolant flow between the grid and the channel. Without the band, there would be excessive flow near the channel which does not contribute to cooling the fuel rods.
In one spacer type, a ferrule surrounds each fuel rod. The outer band has flow tabs along its upper edge. The flow tabs are adjacent the channel wall and hence function to direct some of the steam water mixture onto those fuel rods at the edge. This redirected flow increases the thickness of the water film on the edge rods and increases the critical power for these rods.
Spacers have two adverse effects on fuel bundle performance. First, each spacer is an obstruction to the coolant flow. Consequently, it increases the pressure drop through the fuel bundle. The increase of pressure drop within a fuel bundle is undesirable, especially with modern high density fuel rod arrays exceeding eight by eight and often including arrays ten by ten or denser. Consequently it is desirable to construct the grid portion of the spacers to produce as little pressure drop as possible. However, the thickness of the material from which the ferrule portion or grid portion of the spacer is constructed sets a lower limit on the minimum of flow area obstructed by the spacer.
Secondly, the spacers absorb neutrons. Additional fuel is required to compensate for the reactivity loss resulting from the neutron absorption.